1. Field of the Invention
The invention relates to marine pump jet apparatus.
2. Description of Related Art
Pump jets have been around for a number of years, but have not been widely used. They are generally characterized by a structure which includes a rotor and a stator section all surrounded by a housing. The upstream inlet of the housing is typically larger than the downstream outlet.
Pump jets, in general, have several advantages over traditional exposed propellers. First, because the rotor mechanism is shielded by the housing, it prevents swimmers, water skiers, skin divers, and the like, from being hit and injured by the rotating blades. This can be also important in areas where endangered wildlife, such as manatees, are located. Second, because the rotor is covered, it tends to be less likely to get caught in tow ropes, kelp, seaweed, etc. Third, under certain circumstances, the pump jet is more efficient than traditional exposed propellers. This makes the pump jet especially suitable for sports and military applications. Conventional pump jets are normally mounted as a retro-fit item onto the lower unit of a conventional outboard. This is a relatively simple and straight forward approach because it requires the minimal amount of modification of the outboard marine engine.
A conventional prior art outboard is illustrated in FIG. 1. An antiventilation plate (sometimes referred to as an "anticavitation" plate) is located between the mid-section of the outboard motor and the lower unit. The antiventilation plate prevents the naked rotating propeller from sucking air down from the surface, i.e., aspirating, thereby decreasing the thrust of the propeller. It would be very difficult to place a traditional naked propeller up front of the drive unit because the antiventilation plate would be much less effective in such an arrangement. Therefore, outboard propellers are generally located downstream of the drive unit under the protection of the antiventilation plate. One advantage of pump jets, however, is that they do not need antiventilation plates in view of the fact that the rotor and stator mechanisms are completely covered and protected by a housing.
There has been a moderate amount of effort to develop marine pump jets, even though they have not been widely accepted. Perhaps the best known of the inventors in this area is Dr. Kimball P. Hall whose name appears as inventor or co-inventor on the following U.S. patent which are representative of the state of the art: U.S. Pat. Nos. 3,389,558; 3,849,982; 4,023,353; 5,273,467; and, 5,325,662. All of the foregoing patents describe marine pump jets which are retrofitted onto the lower drive unit of an outboard motor and, therefore, are located downstream of the rotor drive mechanism.
It should be understood that a pump jet is not the same thing as a shrouded propeller. A typical example of a shrouded propeller is disclosed in U.S. Pat. No. 2,473,603. A shrouded propeller is simply a conventional outboard motor propeller surrounded by some form of shroud. A pump jet, on the other hand, has an axial flow pump impeller or rotor, rather than a propeller, because the pump jet is concerned with creating an increase in pressure or head instead of creating thrust. The blades of an axial flow pump rotor are not the same as propeller blades. To maximize the increase in pressure, it is desirable to minimize turbulence. To this end, a pump jet usually has inlet struts and stator vanes to straighten water flow through the pump jet, and the inlet is larger than the outlet.
While tractor pump jets have been employed successfully on outboard marine engines, it would be desirable to provide an arrangement wherein a pump jet can also be conveniently installed on an inboard/outboard or stern drive unit to replace the conventional propeller arrangement as now used.